In breathing apparatus that provide humidification, the intention is to provide humidified air to the patient at a desired humidity and/or temperature to achieve the desired patient comfort and/or therapy. Providing the desired humidity and/or temperature requires achieving and maintaining the humidity and/or temperature of humidified air at the patient (that is, in the vicinity of the patient's mouth and/or nose) as this is the humidity and temperature of air that is ultimately passed to the patient. (NB: “in the vicinity” can also be termed “at or near” or “proximate”.) In theory, this involves heating and/or humidifying air that passes through the flow path of the breathing apparatus from the flow generator to the patient (via e.g. the conduit and patient interface) until the desired humidity and/or temperature proximate the patient is achieved and maintained.
However, while controlling the humidity and/or temperature proximate the patient is possible, it is technically and economically difficult. For example, using sensors to detect the humidity and/or temperature of humidified air proximate the patient is expensive and has technical and health difficulties. Therefore, traditionally it is not the humidity and/or temperature of the humidified air proximate the patient that is controlled, but rather the humidity and/or temperature of humidified air at some other point in the flow path, being a point that is easier to monitor. A target temperature and/or humidity is determined for a particular point (such as the humidifier outlet or the end of the conduit/hose), and it is that temperature and/or humidity that is controlled. The difficulty with this approach is that achieving and maintaining the target temperature and/or humidity at the point in the flow path does not guarantee that the desired humidity and/or temperature are achieved proximate to the patient and delivered to them. The humidity and/or temperature of humidified air can change by the time it reaches the patient. For example, if the temperature and/or humidity of air at the end of the conduit is kept at a target, the humidity and temperature fluctuations provided in the patient interface mean that the actual temperature and/or humidity delivered to the patient via the patient interface varies in a unknown manner from that at the end of the hose. This makes it difficult to know the actual temperature/humidity of air at the patient and makes it difficult to deliver the desired temperature and/or humidity of humidified air to the patient.